This invention relates to a filtering device and, more particularly, to a filtering device suitable for precision filtering in a state submerged in liquid to be treated. Liquid to be treated includes water for general uses, drinking water, river water, pool water and various other liquids.
Known in the art of filtering devices is one which comprises multi-layered filtering materials of different types provided in a single container through which liquid to be treated is subjected to different filtering operations. One example of such prior art filtering device is shown in FIG. 19. In this prior art filtering device, liquid to be treated is introduced into a container a through an inlet b provided in the upper portion of the container a. The liquid to be treated is spread over an upper filtering material layer d through distributors c. The upper filtering material layer d consists of coarse grains of anthracite and impurities of a relatively large diameter in the liquid to be treated are trapped by the anthracite layer. Then the liquid to be treated flows down through the lower filtering material layer e which consists of fine grains of garnet and impurities of a smaller diameter are trapped by the garnet layer. The treated liquid is collected by a collector screen f and discharged from an outlet g provided in the bottom portion of the container a.
Since most impurities in the liquid to be treated are trapped by the upper layer in this prior art filtering device, the anthracite grains of the upper layer tend to be easily blocked by the impurities and, therefore, frequent reverse washing of the upper layer is necessary. For this purpose, compressed air is blown into the upper and lower layers d and e through the collector screen f to stir the filtering material grains of the upper and lower layers d and e and thereby remove the impurities and then reverse washing water is forced into the upper and lower layers d and e through the collector screen f to wash away the impurities. This reverse washing must be performed accurately because otherwise the impurities, once removed from the anthracite grains but left in the container without being washed away, will fall on the surface of the upper layer d and are deposited thereon.
Another problem in this prior art filtering device is that, since the diameter of a filtering material of the upper layer d is generally larger than the diameter of a filtering material of the lower layer e, if the filtering material of the upper layer d is of the same specific gravity as the filtering material of the lower layer e, the filtering material of the upper layer which is of a larger diameter settles sooner than the filtering material of the lower layer e after stirring of the filtering materials made during the reverse washing operation with the undesirable result that the filtering material of the upper layer d is deposited under the filtering material of the lower layer e. For preventing occurrence of this phenomenon, in the prior art filtering device, coarse grains of anthracite which is of a smaller specific gravity than garnet are used in the upper layer d and fine grains of garnet are used in the lower layer e. Thus, selection of filtering materials for the upper and lower layers is limited by the specific gravity of the filtering material.
More importantly, the prior art filtering device has a relatively small effective filtering surface area in comparison with the size of the container in which the multi-layered filtering materials are provided.
It is, therefore, an object of the present invention to provide a filtering device which has a larger effective filtering surface area than the prior art filtering device.
It is another object of the invention to provide a filtering device which is capable of reducing the amount of impurities deposited on the surface of a filtering material and thereby reducing frequency of a reverse washing operation.
It is another object of the invention to provide a filtering device in which selection of two or more different filtering materials can be made freely without being limited by the specific gravity of the filtering materials to be selected.
Other objects and features of the invention will become apparent from the description made heinbelow with reference to the accompanying drawings.
For achieving the above described objects of the invention, there is proivded a filtering device comprising a container having an inlet for introducing liquid to be treated, an annular multi-layered prepacked screen assembly provided in said container and having an outlet for treated liquid, said screen assembly comprising three or more cylindrical screens having different diameters and being coaxially disposed in said container with their axes extending vertically in said container, an annulus being formed between respective adjacent cylindrical screens, and two or more different filter materials performing filtering functions which are different from each other and being filled one in each of the annuluses between the cylindrical screens.
According to the invention, an overwhelmingly larger effective filtering surface area than the prior art filtering device is available owing to the structure of the filtering device as defined in the claim. For example, as will be described more fully later, more than ten times as large effective filtering surface area as the prior art filtering device can be obtained in a case of using a container having a diameter of 2 meters and a height of 2 meters.
Further, since screen assembly is vertically disposed in the container, the liquid to be treated enters the vertically disposed screen surfaces of the cylindrical screens. This allows some part of impurities in the liquid to be treated to fall to the bottom of the container due to gravity before entering the cylindrical screens with the result that the amount of impurities trapped by the filtering materials of the cylindrical screens is thereby reduced. As a result, frequency of a reverse washing operation for removing trapped impurities is reduced as compared to the prior art filtering device in which all impurities of the liquid to be treated are trapped by the filtering materials and the working efficiency of the filtering device can thus be improved. Impurities which have settled on the bottom of the container can be removed periodically by opening a drain port provided in the bottom of the container.
Since the filtering materials are prepacked in the cylindrical screens of the annular multi-layered prepacked screen assembly in the form of a cartridge, replacement of the filtering materials by new filtering materials can be made simply by replacing the entire screen assembly and this replacement work will be much easier and less time consuming than in the case of the prior art filtering device in which the filtering materials must be replaced one layer after another.
Since the two or more different filtering materials are partitioned from each other by the cylindrical screens, there is no likelihood of mixing of the different filtering materials with each other during the reverse washing operation. Accordingly, it is not necessary to select, as in the prior art filtering device, filtering materials of different specific gravity for preventing reverse resettling of the filtering materials and, therefore, any desired filtering materials of different filtering functions can be combined freely without considering the specific gravity of each filtering material. This will enable a combination of filtering materials which will achieve a more efficient filtering result at a reduced cost.
Furthermore, since impurities removed from the filtering materials by the reverse washing operation will be completely washed away or fall to the bottom of the container due to gravity and will never return to the surfaces of the filtering materials, efficiency of the reverse washing operation will be improved as compared to the case of the prior art in which some of impurities removed from the filtering materials tends to return to the surface of the filtering material on the upper layer due to gravity.
In one aspect of the invention, each of said cylindrical screens of the screen assembly has a closed cylindrical wall portion extending from one end of the cylindrical screen to form an annular chamber between the respective adjacent cylindrical screens, said annular chamber being continuous to said annulus between the adjacent cylindrical screens filled with the filter material and being unfilled with the filtering material and permitting free upward movement of the filter material within said annular chamber during reverse washing.
According to this aspect of the invention, the filtering materials can move upwardly within the annular chambers during reverse washing whereby turning over of the filtering materials filled in the entire annuluses between the cylindrical screens during reverse washing is permitted and removal of impurities from the filtering materials can thereby be facilitated.
In another aspect of the invention, a plurality of said annular multi-layered screen assemblies are provided in the container and the outlets of said screen assemblies being connected with a common outlet pipe.
In another aspect of the invention, a filtering device comprises a plurality of such filtering devices which are disposed in parallel, the inlets of liquid to be treated and the common outlet pipes of said filtering devices being connected respectively with common connecting pipes to form a module.
In another aspect of the invention, a filtering device comprises a plurality of such filtering devices forming a module, the common connecting pipes of said filtering devices being further connected respectively with common connecting pipes to form a larger module.
In another aspect of the invention, an innermost screen of said screen assembly comprises support rods extending in the axial direction of the screen and arranged generally cylindrically with a predetermined interval, and a cylindrical filter member attached fixedly on the outside of the support rods, and said filtering devices further comprises a vibrator including a shaft provided in the central portion of the screen coaxially with the screen and at least one leaf spring which is fixed in one end portion thereof to the shaft and is capable of abutting in the other end portion against a part of each of the support rods, and a drive unit for rotating at least one of the screen assembly and vibrator.
According to this aspect of the invention, by rotating at least one of the screen assembly and the vibrator, one end portion of the leaf spring sequentially abuts against each of the cylindrically arranged support rods and is disengaged therefrom and, by the impact of the leaf spring abutting against the support rods, slight vibration occurs in each support rod and the filter member which is fixed thereto. Powdery suspending solid particles of impurities in the liquid to be treated which tend to be deposited on the surface of the filter member as the liquid enters the filter member are shaken off from the surface of the filter member by this vibration, so that these suspending solids are kept away from the surface of the filter member whereby blocking of the filter member can be prevented. The filtering device therefore can prevent blocking of filter elements while continuing the filtering operation without stopping the filtering operation for reverse washing.
In another aspect of the invention, an outermost screen of said screen assembly comprises support rods extending in the axial direction of the screen and arranged generally cylindrically with a predetermined interval, and a cylindrical filter member attached fixedly on the inside of the support rods, and said filtering device further comprises a vibrator including a frame provided on the outside of the screen coaxially with the screen and extending in the axial direction of the screen and at least one leaf spring which is fixed in one end portion thereof to the frame and is capable of abutting in the other end portion against a part of each of the support rods, and a drive unit for rotating at least one of the screen assembly and vibrator.
According to this aspect of the invention, a similar effect of shaking off of solid particles of impurities by vibration can be obtained.
In another aspect of the invention, a filtering device further comprises flow creating means for creating, in said container, a flow of the liquid to be treated having a direction which is different from direction of a flow of the liquid to be treated into the screen assembly.
According to this aspect of the invention, a flow of the liquid to be treated having a direction which is different from direction of a flow of the liquid to be treated into the screen is created and powdery suspending solid particles of impurities in the liquid to be treated which tend to be deposited on the surface of the screen as the liquid enters the screen are washed away from the surface of the screen by this flow having the different direction from the flow into the screen, so that these suspending solid particles are kept away from the surface of the screen whereby blocking of the screen can be prevented. The filtering device therefore can prevent blocking of filter elements while continuing the filtering operation without stopping the filtering operation for reverse washing.
In another aspect of the invention, a filtering device further comprises a reverse washing tube provided coaxially in an innermost screen of said screen assembly and rotatably about the axis thereof, said reverse washing tube having an inlet for the treated liquid in one end portion thereof and an opening for discharging the treated liquid and introducing reverse washing water in the other end portion thereof and having a plurality of reverse washing water injecting holes at a predetermined interval in the axial direction of the tube, a liquid transfer tube communicating with the opening for discharging the treated liquid and introducing reverse washing water, a check valve provided on the side of the opening for discharging the treated liquid and introducing reverse washing water in the vicinity of the inlet for the treated liquid of said reverse washing tube for permitting flow of the treated liquid from said inlet into the reverse washing tube and prohibiting flow of the reverse washing water from inside of the reverse washing tube to said inlet, and means for rotating the reverse washing tube about its axis.
According to this aspect of the invention, by injecting pressurized reverse washing water from the reverse washing water injecting holes of the rotating reverse washing tube, reverse washing water is jetted out in a jet stream and this jet stream sequentially collides with the entire inner peripheral walls of the cylindrical screens thereby uniformly removing the impurities blocking the cylindrical screens.
In still another aspect of the invention, an outermost screen of said screen assembly is a wedge wire screen having a screen slit formed on the outside of the screen and having an opening which widens radially inwardly from the screen slit.
Preferred embodiments of the invention will be described below with reference to the accompanying drawings.